Shirk is the worst sin in Islam. Allah may forgive any sin if one dies in that state except for committing shirk. What is shirk?

In Islam, shirk is the sin of practicing idolatry or polytheism, i.e., the deification or worship of anyone or anything besides the singular God, i.e., Allah. Literally, it means ascribing or the establishment of "partners" placed beside God. Those who practice shirk are termed mushrikun. Within Islam, shirk is an unforgivable crime if it remains unpardoned before death: Allah may forgive any sin if one dies in that state except for committing shirk.